With the general improvement in plastics materials, it has become customary to produce a fan volute from moulded plastics material, usually in two halves which are interconnected, and to provide a plurality of radiating arms carrying the bearings for the central shaft of the impeller (see for example our Australian Pat. No. 536805). The shaft is usually journalled in bearings which are supported in a bearing housing carried by the radiating arms, and having a ring of elastomeric material interposed to absorb the high frequency vibrations.
However in the moulding of plastics material, difficulty is sometimes encountered due to the large co-efficient of expansion of the material, and the fact that the material is hot when moulded. Consequently variation of wall thickness can result in distortion of shape when a moulded plastic part of large size and relatively thin wall is cooling after having been moulded. Accordingly it has been the custom in the past to utilise metal radiating arms secured to securing pads on the fan casing volute by fasteners, the most commonly used fasteners being metal thread screws and nuts. While this overcomes a distortion problem, it introduces a secondary problem in that high stress areas are created at the localities of the fasteners.
One object of this invention therefore is to provide improvements in fan casing volutes wherein difficulties due to differential contraction of the plastics material, and difficulties due to high stress areas, are substantially reduced.